A MC is used in conjunction with an AC dynamoelectric machine to provide variable and controllable speed for a multiplicity of applications. It converts a DC power source to polyphase AC of suitable power and frequency to drive the AC dynamoelectric machine for such applications.
In aircraft, a MC has uses for both low power and high power applications. One high power application is main engine starting, which may be accomplished with a MC and a dynamoelectric machine that functions as a starter motor and a main electrical power generator. Another such aeronautical high power application is the use of a MC for an electric motor driven hydraulic pump (HP). In aircraft, it is common practice to use the main engine starting MC to serve another function after the engine is started, such as controlling other motor drives, also requiring high power ratings.
It is common for the HPs and other aeronautical motor drives to operate a very high rotational speed to minimize size and weight of the motor. Associated with this high speed is a relatively high frequency required from the MC. Speeds of 42 krpm and 84 krpm are not unusual with operating frequencies up to 1300 or 1400 Hz. On the other hand, at the associated power levels of 100 kW to 200 kW needed for these applications, the switching frequencies for the MC power transistors, typically insulated gate bipolar transistors (IGBTs), are typically limited to about 10 kHz maximum.
It is well known that motors need the low order harmonic potentials and currents up to about the 7th harmonic minimised or eliminated to provide efficient operation of the motor. With switching inverter type motor drives, it is also known that control of frequency components in terms of being able to produce or eliminate them requires a switching frequency at least two times the highest frequency of concern. Thus, for effective minimisation of the 7th harmonic of 1400 Hz motors, 9800 Hz, we need about 20 kHz for the switching frequency. Operating IGBTs at this frequency and the requisite power levels will result in unacceptable losses. Ten kHz is considered an acceptable maximum switching frequency for these applications.
Another requirement for airborne applications is filtering of the MC's input and output. In the non-aeronautical industrial field, it is customary to filter the input, but not the output, of the MC. This is because the MC is generally dedicated to a single motor and is either located in extremely close proximity to the motor, or shielding over the wiring between the MC and motor is used. In aircraft applications, the MC may be required to serve more than one motor and/or located at relatively large distance from the motor. Shielding of the MC-to-motor wiring is generally not acceptable because of increased weight and increased heating in the wiring caused by the close bundling and reduced air circulation that shielding causes.
It is known that filtering applied to the output of the motor drive is detrimental to the system performance because it inherently extracts a portion of the fundamental torque producing potential and current delivered to the motor. It is also generally accepted that higher MC switching frequencies result in an output filter design that reduce the loss of fundamental potential and current delivered to the motor. It is further known if the MC inverter switching frequency is insufficient to eliminate the 5th and 7th harmonics potentials, then the addition of an output filter is not a practical solution and it can increase the presence of those unwanted harmonics.